The restoration of teeth commonly involves a light curable dental composition containing free-radically polymerizable resins. Light curing of a dental composition involves a photoinitiator system generating free radicals upon exposure to visible light. Free radicals may be typically produced by either of two pathways:    (1) the photoinitiator compound undergoes excitation by energy absorption with subsequent decomposition of the compound into one or more radicals (Norrish type I), or    (2) the photoinitiator compound undergoes excitation and the excited photoinitiator compound interacts with a second compound by either energy transfer or a redox reaction to form free radicals from any of the compounds (Norrish type II).
In order for a photoinitiator to be useful for use in a dental composition, the quantum yields indicating the conversion of light radiation to radical formation needs to be high since absorption or shielding of light by further components of the dental composition limit the amount of energy available for absorption by the photoinitiators. Accordingly, only about 70 percent conversion of the polymerizable groups may be expected in a polymerization of a typical dental composition initiated for example by a camphor quinone (CQ)/amine photoinitiator system, whereby the mechanical strength of the polymerized dental composition is less than optimal and unreacted monomers may leach out of the polymerized dental composition. The leaching monomers may have detrimental effects. In order to alleviate this problem, multifunctional monomers are frequently used which are more likely to be included in the polymer network.
In addition, photoinitiators are required to have a high acid resistance, solubility (e. g. for adhesives), thermal stability, and storage stability when incorporated into a dental composition.
Finally, given that dental compositions usually contain (meth)acrylate or (meth)acrylamide monomers, free radical photocuring may be inhibited by the presence of oxygen. Oxygen inhibition is due to the rapid reaction of propagating radicals with oxygen molecules to yield peroxyl radicals which are not as reactive towards carbon-carbon unsaturated double bonds and therefore do not initiate or participate in any photopolymerization reaction. Oxygen inhibition may lead to premature chain termination and, therefore, incomplete photocuring. Nevertheless, a certain degree of oxygen inhibition on the top surface of the adhesive layer is required for the bonding to the adjacent restorative.
Accordingly, the polymerization initiator system has a critical influence on the quality of the dental material. Conventionally, camphor quinone in combination with a tertiary amine and optionally with a diphenyliodonium (DPI) salt or 2,4,6-trimethylbenzoylphenyl phosphinate (Irgacure® TPO) are frequently used as photoinitiator system. However, the presence of amines in acrylate-containing compositions can cause yellowing in the resulting photocured composition, and soften the cured composition. Moreover, the use of aromatic amines might give rise to toxicological concerns.
Furthermore, it is desirable that the light activating the photoinitiator system has a long wavelength in order to avoid damage of soft tissue during polymerization of the dental composition in the patient's mouth. Accordingly, the photoinitiator system is required to contain a chromophoric group efficiently absorbing light of the desired wavelength in a range of from 400 to 800 nm. Accordingly, it is important to find a photoinitiator system with a sufficiently high absorption coefficient. However, the absorbtion coefficient must not be excessively high, because an excessively high absorption coefficient of the photoinitiator system increases the coloration of the photoinitiator system and thereby the coloration of the dental composition before light curing. In any case, it is necessary that the chromophoric groups are efficiently destroyed during polymerization so that the coloration of the initiator system disappears in the polymerized dental composition, the so-called “photo-bleaching”. A destruction of the chromophoric groups during polymerization may also be useful in increasing the depth of cure of the dental composition since activating light is not shielded from unpolymerized layers of the dental composition by the photoinitiator system present in polymerized layers covering the unpolymerized layers.
EP 0 076 102 A1 discloses a photopolymerizable composition comprising an epoxy compound, a curing catalyst including at least one aluminium compound having at least one organic radical directly bonded to the aluminum atom, at least one α-ketosilyl compound and at least one photosensitizer selected from the group consisting of benzophenone compounds and thioxanthone compounds. The photopolymerizable composition may be used in the field of electrical equipment, e.g. for producing an insulating material, or as a photoresist material.
EP 1 905 415 A1 discloses dental compositions comprising a polymerizable binder and a photoinitiator containing an acylgermanium compound.
EP 2 103 297 A1 discloses compositions, among others dental compositions, which comprise at least a polymerizable binder and a polymerization initiator comprising at least an acylgermanium compound. The acylgermanium compound comprises 2 to 100 acylgermanium moieties, which are linked via a bond or a branched or linear aliphatic, aromatic or aliphatic-aromatic hydrocarbon residue having a valency corresponding to the number of acylgermanium moieties. As a reference example, a dental composition is disclosed comprising a polymerization initiator system consisting of benzoyltrimethylgermane, and the polymerizable compounds UDMA and triethyleneglycoldimethacrylate.
US 2015/0080490 A1 discloses a polymerizable dental composition comprising a photoinitiator mixture which contains at least one diacylgermanium compound such as bis-(4-methoxybenzoyl)diethylgermanium, at least one α-diketon such as camphor quinone, and an accelerator.
WO 2015/144579 A1 discloses a polymerizable dental composition comprising a photoinitiator mixture that contains an α-diketone photoinitiator compound having a light absorption maximum in the range from 300 to 500 nm such as camphor quinone, and a coinitiator in the form of a hydride of silicium or germanium.
El-Roz M. et al., Current Trends in Polymer Science, 2011, vol. 15, pages 1 to 13 discloses free radical photopolymerisation of an epoxy acrylate monomer in the presence of a photoinitiator system consisting of isopropylthioxanthone as photoinitiator in combination with acylsilane compounds, among others methyl(trimethylsilyl)methanone and methyl(triphenylsilyl)methanone. This document does not disclose dental compositions. In the above described conventional dental compositions comprising silicium or germanium containing organic compounds as photoinitiator, typically, a coinitiator is contained. For example, EP 1 905 415 A1 and US 2015/0080490 A1 disclose ethyl N,N-dimethylaminobenzoate (EDB) as coinitiator.